(a) Technical Field
The present invention relates to a brake system for a vehicle, and more particularly, to a hydraulic booster brake system, which includes a pedal simulator for generating a pedal feel based on an engagement degree of the driver, to generate hydraulic pressure required for braking based on a braking intent of the driver, and to supply the generated hydraulic pressure to a wheel brake.
(b) Background Art
Recently developed vehicles utilize brake-by-wire technology using an electronic control system, for control of braking, in place of general hydraulic control systems known in the art. In such an electronically controlled brake system, hydraulic pressure required for braking based on a braking intent of the driver is generated through use of an electric motor. The hydraulic pressure generated based on driving of the motor is transferred to a wheel brake (wheel cylinder) of each wheel and, accordingly, a braking force is generated. Such an electronically controlled brake system, which controls hydraulic pressure, using an electronic actuator, is typically referred to as an “electro-hydraulic brake system (EHB)”, namely, an “electronic hydraulic brake system”.
The electronic hydraulic brake system controls braking forces generated at respective wheels in an independent manner Accordingly, it may be possible to realize functions associated with, for example, electronic stability control (ESC) or anti-lock brake system (ABS). A pump is often used as an electronic actuator, which is used in a general hydraulic booster brake system, namely, an electronic hydraulic brake system as mentioned above.
The pump converts rotating force generated during driving of a motor into linear force to move a piston in forward and backward directions. In accordance with the movement of the piston, brake fluid in a chamber of a cylinder is pressurized, thereby generating hydraulic pressure.
In this electronic hydraulic brake system, a sensor is configured to sense a pedal stroke generated based on pedal depression (e.g., engagement degree) of the driver, and the pump generates hydraulic pressure based on the sensed results through driving of the motor, and, as such, braking forces of respective wheels are controlled. In addition, the electronic hydraulic brake system is equipped with a pedal simulator that enables the driver to perceive a pedal feel as in a general hydraulic brake system. In particular, when the driver depresses or engages a pedal connected to a master cylinder, hydraulic pressure of brake fluid in a master cylinder is increased. The hydraulic pressure from the master cylinder is transferred to the pedal simulator via a pedal-side hydraulic line and, as such, a pedal feel is generated from the pedal simulator.
In addition, in response to depression of the brake pedal by the driver, a controller is configured to calculate a target hydraulic pressure desired by the driver, based on a driver's pedal input (e.g., braking input) sensed through a brake pedal sensor (e.g., pedal stroke sensor), namely, a pedal stroke value. Based on the calculated target hydraulic pressure, the controller is configured to drive the motor, to cause the pump to generate hydraulic pressure. The hydraulic pressure generated from the pump is transferred to each wheel cylinder and, as such, a desired braking force is obtained.
Meanwhile, in a conventional electronic hydraulic brake system, the side of a master cylinder and the side of a pedal simulator are connected via a pedal-side hydraulic line. In addition, in such a conventional electronic hydraulic brake system, operation of a motor in an electronic actuator is stopped when the system fails. Particularly, a cut valve is opened, to allow hydraulic pressure generated from the master cylinder based on a foot effort of the driver to be directly transferred to the wheel brake (wheel cylinder). Thus, braking force is generated by the hydraulic pressure of the master cylinder.
In connection with this, for an increase in the braking force generated by the foot effort of the driver when the system has failed, it is necessary to reduce the inner diameter of the master cylinder. However, when the inner diameter of the master cylinder is reduced, the hydraulic pressure at the side of the pedal simulator connected to the master cylinder via the pedal-side hydraulic line may be increased at a given pedal stroke. Particularly, durability of elements of the pedal simulator, for example, a spring and a rubber damper, may be insufficient.
For an increase in durability of the spring and damper of the pedal simulator, it is necessary to increase the size of the pedal simulator. However, there is a limitation in increasing the size of the pedal simulator due to an increase in seal friction of the pedal simulator and a limited engine compartment layout space. In this regard, reducing the inner diameter of the master cylinder has a limitation in enhancing a backup braking performance, namely, a function of generating braking force by hydraulic pressure generated from the master cylinder by a foot effort (e.g., an engagement degree) of the driver in an opened state of the cut valve when the system has failed.